Ubiquitous computing (ubicomp) environments provide access to information and computing resources for users at any time and anywhere. In the approaching ubiquitous ear, many self-adaptive applications are emerging. These can be made to adapt to the environment within which the application operates. Context information is the key to producing self-adaptive applications in ubicomp environments. However, the supporting infrastructure that generates context information can be made dynamically responsive to the environment. Therefore, for the development of self-adaptive applications, it is necessary to demonstrate that valid context information can be created by virtual sensors instead of physical sensors. In this paper we present a context aware simulation system called CASS. In particular, it generates the context information associated with virtual sensors and virtual devices in a smart home domain. By using CASS, the self-adaptive application developer can immediately detect rule conflict in context information and determine optimal sensors and devices in a smart home.